1. Field of the Invention
The present invention relates to a CDMA communication system, and in particular, to a handoff method in a CDMA system.
2. Background of the Related Art
In communications systems having geographically fixed stations and mobile stations, it is necessary to handoff the communications with the mobile station from a first fixed station to a second fixed station, as the mobile station moves farther away from the first fixed station and closer to the second fixed station. In the cellular communications context, as a mobile station travels out of a first cell and into a second cell, communications with the mobile station must be passed from a base station in the first cell to the base station in the second cell.
There are three types of conventional handoffs, depending on the multiple access system employed in the communications system. They are the hard handoff, soft handoff, and softer handoff.
A hard handoff is characterized by a temporary disconnection of the forward and reverse channels and is typical in a FDMA or TDMA environment. As the mobile unit moves out of a currently serviced cell area during a communications session, the received signal becomes weak and a handoff is required. To perform the hard handoff, the communications system switches the communications session to a new channel while the session continues. In hard handoffs in FDMA or TDMA (or from one CDMA system to another CDMA system using a different frequency or frame offset), the receiver in the mobile unit stops demodulating and decoding information transmitted on the old channel link, from the cell initially servicing the session, and then starts demodulating and decoding information transmitted via a second channel link.
In a FDMA or TDMA cellular system, the handoff scheme implemented is intended to allow a call to continue when a mobile telephone crosses the boundary between cells. The handoff from one cell to another is initiated when the receiver in the cell base station handling the call notices that the received signal strength from the mobile station falls below a predetermined threshold value. A low signal to noise ratio indication indicates that the mobile telephone is on the cell boundary. When the signal level falls below the predetermined threshold value, the base station asks the system controller to determine whether a neighboring base station is receiving the mobile signal with better signal strength than the current base station. The system controller, in response to the current base station's inquiry, sends messages to the neighboring base stations with a handoff request. The base stations neighboring the current base station employ scanning receivers which receive the signal from the mobile station on the specified channel.
A handoff will be attempted when one of the neighboring base stations reports an adequate signal level to the system controller. This scenario is called a “base station initiated handoff process.” Handoff is then initiated when an idle channel from the channel set used in the new base station is selected. A control message is sent to the mobile station commanding it to switch from the current channel to the new channel. At the same time, the system controller switches the call from the first base station to the selected base station. This process is sometimes called “break before connect.” Because a hard handoff is completed by a temporary disconnection of the traffic channel, information in the received signal may be lost.
The soft handoff (as used in a CDMA environment) alleviates the problem of the temporary disconnection. In a soft handoff, two or more received signals through different cells are simultaneously demodulated, combined, and decoded by the same receiver unit. Because the CDMA environment enables the receiver to simultaneously demodulate, combine, and decode signals from more than one base station, the soft handoff does not require any disconnection of the traffic channels. A user moving into the service area of another base station does not need to change its receiving or transmitting frequency. A soft handoff is characterized by initiating communications using a new code sequence (i.e., with a new base station at a new cell) on the same CDMA frequency before terminating communications with the old code sequence.
One soft handoff system used in conjunction with a cellular communication system is described in U.S. Pat. No. 5,640,414 for the “Mobile Station Assisted Soft Handoff in a CDMA Cellular Communications Systems,” issued to Blakeney II, et al. (the “414 patent”), which is incorporated herein by reference. The initiation of the handoff process is invoked by the mobile station measuring the signal power of the handoff-assisting pilot signal over the pilot channel in the CDMA system. Alternatively, it can be invoked by the base station measuring the signal power from the mobile station. A typical CDMA soft handoff is implemented by diversity combining (i.e., combining signals from either the same or different base stations) in conjunction with a RAKE receiver, thereby providing better call reliability than a hard handoff. Additionally, the handoff process between cells is completed in a manner that is transparent to the user.
As described in the 414 patent with reference to a cellular communications system, the mobile initiated handoff method is different from the base station initiated handoff method. The mobile initiated handoff relies on the mobile station to detect the presence or absence of pilot signals and the signal strength of the pilot signals. Thus, in order to perform a handoff initiated by a mobile station, the mobile station is equipped with a search receiver to scan pilot signals from other base stations. One reason to employ a mobile initiated handoff method is that the mobile station is more sensitive than base stations to changes in path between itself and various neighboring base stations.
In a related art CDMA system, two types of handoff operations are typically implemented. They are the soft handoff and the CDMA-to-CDMA hard handoff. The CDMA-to-CDMA hard handoff is similar to that of the TDMA or FDMA system, and call interruption may occur. Each of these methods is described in more detail below.
In the soft handoff situation, the mobile station typically initiates the handoff process. The mobile station performs signal diversity combining to and/or from multiple base stations. The mobile station employs RAKE receivers to receive communications simultaneously from the multiple base stations. A soft handoff occurs when the mobile station is communicating simultaneously with two or more base stations, or with two or more sectors of the same base station (softer handoff) before communications with the previous base station or sector is dropped. The softer handoff is a special type of soft handoff, and no distinction is made herein between a soft and a softer handoff. In the soft handoff environment, the call between a mobile station and an end user is not interrupted by the eventual handoff from the base station corresponding to the cell from which the mobile station currently is being serviced to the base station corresponding to the cell from which the mobile station is to receive services.
Generally in a handoff, a mobile station currently in communication continuously probes the intensity of pilot signals from the currently connected source base station and from neighboring base stations or sectors. It then compares the intensity of the pilot signals from the probed base stations with the intensity of the pilot signal of the current source base station, and reports to the source base station for a handoff to one of the neighboring base stations when the intensity of the pilot signal from the neighboring base station exceeds a predetermined value. The source base station orders the mobile station to perform a handoff when prepared to perform handoff to the corresponding base station or sector. Therefore, the mobile station can continuously communicate while obtaining a new communication channel.
In the CDMA Cellular/Personal Communication Service (PCS) telephone system, a mobile station uses a soft handoff that supports a new communication channel while maintaining the previous communication channel when performing a handoff between base stations or sectors having the same frequency, frame offset, and exchanger. However, the mobile station uses a hard handoff when it moves to a base station that belongs to a different switching center, changes the traffic channel into another frequency channel, or switches into another traffic channel having a different frame offset.
In turn, the third generation partnership project 2 (3GPP2) supports a new handoff different from the foregoing handoff since a concurrent multiple service is supposed to support a voice data service together with a packet data service.
In particular, when a mobile station in which voice data is being served and a packet data service is in a dormant state performs a hard handoff into an area where a packet zone identifier (PZ—ID) is changed, the mobile station can not monitor a paging channel in a traffic channel occupying state. Therefore, the mobile station cannot recognize the PZ—ID change, and thus fails to initiate a dormant handoff. This can present a problem. For example, when the packet data service is resumed after completion of the hard handoff, a packet control function (PCF) and a packet data serving node (PDSN) will attempt service to the source base station instead of the target base station where the mobile station is actually located. Consequently, the packet data service cannot be provided to the corresponding mobile station.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.